This invention relates to an intruder detection system capable of convertly sensing and indicating the entry point of an intruder into a selected area, a factory site for example.
Intruder detection systems are known in which two wires comprising a transmitting wire and a receiving wire are buried together around the perimeter of a site to be protected. Radio frequency or microwave signals are sent out along the transmitting wire from a control point and received back via the receiving wire. Crossing of the two wires at any point by an intruder causes an increase in coupling between them and a consequent increase in the signal received at the control point. Disadvantages of such systems are that two wires need to be buried, the coupling between them varies with their length and with soil conditions (which vary hour by hour) making the nature of a disturbance difficult to determine, and the transmitted signal is both detectable and can be interfered with by the intruder.
Optical fibres are known for use in covert and secure transmission systems. In such systems light signals are transmitted along the length of a cylindrical transparent core encased by an integral cladding of lower refractive index than the core, which cladding maximises total internal reflection of the light signals at the core/cladding interface, thereby to minimise signal loss from the core. A known deficiency of optical fibres having an abrupt refractive index step between the core and the cladding, i,e., stepped-index fibres, is the loss of light that can occur through the cladding if the fibre is subjected to localised bending sufficient to cause a decrease in the angle of incidence of the light at the core/cladding interface in the region of bending, to less than the critical angle for total internal reflection, herein referred to as `micro-bending`. This effect is usually minimised in optical fibres for telecommunication purposes by the use of more expensive graded-index fibres having a progressively reducing refractive index from the central core to the cladding, thus avoiding abrupt transitions.